Abstract

Introduction

In lung fibrosis, alveolar epithelium degenerates progressively. The goal of regenerative
medicine is to aid repair and regeneration of the lost tissues in parenchyma and airways
for which mobilization of tissue-resident endogenous or bone marrow-derived exogenous
stem cells niches is a critical step. We used a lung injury model in mice to identify
and characterize functional lung stem cells to clarify how stem cell niches counteract
this degenerative process.

Results

STA- In lung, cellularity increased by 5-fold in WT and 6-fold in NOX-/- by d7. Lung
epithelial markers were very low in expression in all SP flow sorted from lung of
all three genotypes cultured ex vivo. (p < 0.01). Post-bleomycin, the SP in NOX-/- lung increased by 3.6-fold over WT
where it increased by 20-fold over controls. Type I and II alveolar epithelial cells
progressively diminished in all three genotypes by d21 post-bleomycin. D7 post-bleomycin,
CD45+ cells in BALf in NOX-/- was 1.7-fold > WT, 57% of which were Mf that decreased
by 67% in WT and 83% in NOX-/- by d21.LTA- Cellularity as a factor of time remained unchanged in BM, PB, LP and BAL fluid.
BrdU+ (LRC) were the putative stem cells. BrdU+CD45+ cells increased by 0.7-fold and SPC+CC10+ bronchoalveolar stem cells (BASC), decreased by ~40-fold post-bleomycin. BrdU+VEGF+ cells decreased by 1.8-fold while BrdU-VEGF+ cells increased 4.6-fold. Most BrdU- cells were CD45-. BrdU- BASCs remained unchanged post-bleomycin. CFU-c of the flow-sorted BrdU+ cells remained similar in control and bleomycin-treated lungs.

Conclusion

STA- Inflammation is a pre-requisite for fibrosis; SP cells, being the putative stem
cells in the lungs, were increased (either by self renewal or by recruitment from
the exogenous bone marrow pool) post-bleomycin in NOX-/- but not in DKO indicating
the necessity of cross-talk between gp91phox and MMP-12 in this process; ex vivo cultured SP progressively lose pluripotent markers, notably BASC (SPC+CC10+) - significance
is unknown. LTA- The increase in the hematopoietic progenitor pool in lung indicated that exogenous
progenitors from circulation contribute to lung regeneration. Most non-stem cells
were non-hematopoietic in origin indicating that despite tissue turnover, BASCs are
drastically depleted possibly necessitating recruitment of progenitors from the hematopoietic
pool. Loss of VEGF+ LRC may indicate a signal for progenitor mobilization from niches. BrdU- BASC population may be a small quiescent population that remains as a reserve for
more severe lung injury. Increase in VEGF+ non-LRC may indicate a checkpoint to counterbalance the mobilization of VEGF+ cells from the stem cell niche.